It is quite common on construction sites to use devices for detecting underground structures before or while digging. Such structures are occurring often in form of services for supplying electricity, gas, fuel, water, or communication data among other conductive, underground structures. Although the location of most of these services is already known from a surveyor's plan of the site, their locations can have uncertainties or there could be additional services that are not mentioned therein. Often they are also simply overlooked by the operator of an earth moving machine during work.
The avoidance of damage to underground structures while digging in a trench or in areas being excavated is an important task. As damage to a service can cause serious impact and costs, these additional measurements are taken to be able to detect the proximity of such services on the site before or while excavating. Devices for this purpose are known as Cable Avoidance Tools also called CAT.
One way to locate underground services is to detect electromagnetic fields sent out by the service itself. To do this the service requires having a naturally occurring electrical signal witch emits a field that is detectable above the ground. This works well for switched on power supplies, but for example a wiring system of switched off street lights, unused or low-voltage communication cables, gas- or water-pipes can hardly be detected in this way.
In order to avoid these drawbacks, several developments were made to improve the detection and to be able to detect different types of services as well. U.S. Pat. No. 5,194,812 shows a solution for detecting hollow pipes like gas or water pipes by introducing a conductor into them—or by laying a conductor next to the service—that will function as a transmitter for electromagnetic fields. In this way also a pipe not having a naturally occurring field by itself can be detected by an electromagnetic field as indicated above. A drawback of this method is that it isn't always possible to insert such a transmitter into the pipe. Often it happens that a transmitter can not be inserted since the outlet of the service isn't accessible, far away from the site or the service is currently in use and therefore not accessible.
If the service itself is electrically conductive, another way to emanate a field can be used. U.S. Pat. No. 4,438,401 discloses a system wherein metallic services with no naturally occurring signals are directly connected to a signal-generator. In this way an electrical signal can be coupled to the service, and therefore it is possible to detect it by its electromagnetic field. For doing so the service has to be located and connected somewhere near the construction site. This can be quite a difficult task, since some services are hidden, not accessible or even live and therefore can not be connected to.
Another possibility for applying a signal to a service is to emit an electromagnetic field into the ground by a transmitting aerial. A receiving aerial picks up the signals reflected back by the underground structure and based on this information the proximity of a service is determined. U.S. Pat. No. 4,600,356, U.S. Pat. No. 5,592,092 and U.S. Pat. No. 6,437,726 show such devices that are for example mounted on the bucket of a backhoe. These systems work well for lots of metallic services, but the detection is still limited to services containing electrically conducting materials. For example, water-conduits made out of plastic are hardly detectable in this way.
It is also possible to detect underground pipes by other means than an electromagnetic field. U.S. Pat. No. 5,617,031 for example shows a detection method based on the analysis of the impedance of the ground with three ground-conducting contacts. The presence of a structure can be concluded according to the change of impedance between the contacts while dragging the contacts along the ground. The accuracy of the detection is dependent on the skills of the operator who carries out the method and is in general quite low. The measurement requires dragging of the conductive contacts along the ground. Therefore, such a device can not be used on uneven grounds or when the surface is covered, for example, by gravel or tar. It can, furthermore, not be mounted directly to an excavator to assist the operator during work.
What most of the known and used systems have in common is that the underground structures need to emit electromagnetic fields within a usable frequency range that are strong enough to be detectable above the surface.
The fields can be caused by naturally occurring electrical signals which limits the usability mostly to live power lines and some communication cables.
The method of sending out electromagnetic fields and determining the presence of a structure according to the field reflected back from the structure is limited in its performance due to the quiet low signal strength achievable by this. The alternative of externally applying a signal of desired frequency to the structure directly by conduction is often not possible.